PROJECT SUMMARY In our previous grant period we successfully developed a high throughput screening (HTS) assay for thyroid stimulators and discovered a series of small molecule TSH receptor (TSHR) agonists; two of which we have fully characterized. More recently we have developed an HTS assay for TSHR antagonists and have begun to characterize our lead molecules. In addition, we have successfully pursued the role of antibodies to the TSHR cleavage region and shown them to be effective initiators of thyroid cell apoptosis when not opposed by thyroid stimulators and have clarified the control of thyroid cell stress in autoimmune thyroid disease. This competitive renewal proposal has 3 specific aims: Specific Aim 1: To identify and characterize TSHR agonist small molecules with a signaling bias and to dissect the consequent TSHR signaling responses. Specific Aim 2: To fully characterize the activity of our recently identified small molecule TSHR antagonists which have high therapeutic potential. Specific Aim 3: To perform new in vivo studies of antibodies to the TSHR cleavage region (C-TSHR-Abs). Small molecules can be low cost to produce, can be taken orally and are highly specific ? all characteristics which would make their use in thyroid dysfunction a great clinical asset as well as important investigational tools. In addition, the evaluation of the role of antibodies to the cleavage region of the TSHR has already provided insight into the complex immune response to the TSHR and our data now suggest they may be involved in the perpetuation of the autoimmune reaction to the TSHR and the chronicity of the disease ? an area we wish to explore further.